Power consumption of portable, battery-operated electronic devices has presented a significant problem to designers of such circuits since this parameter determines the recharge frequency and available operating period as a result of a single charging of the battery. For example, in mobile battery operated telephones, the available talk time between a battery charging is one of the most competitive marketing factors. The industry is therefore constantly attempting to minimize power consumption in the battery-operated telephonic device.
In portable battery-operated systems which use amplifiers to drive acoustic output devices, such as, for example, an earphone or speaker, a significant amount of power is required to drive such acoustic output devices. This power is consumed by the amplifiers which generally operate in typical class AB wherein the power efficiency is Voutput/VDD (assuming no quiescent power consumption). On the other hand, while class D amplifiers have a one hundred percent power efficiency (assuming an ideal switch) for the single-ended signal input, class D amplifiers also require the use of an inductor as a filter to limit the high frequency current in the system. The radiation from the inductor has been found to provide RF interference with the operation of portable battery operated wireless communication equipment and therefore cannot be used in conjunction with such systems. On the other hand, for a differential system, if the inductor is eliminated from the class D amplifier, the class D amplifier efficiency becomes about (Voutput/VDD).sup.2 which is generally less than class AB amplifier efficiency. Accordingly, a class D amplifier is not a viable alternative to the class AB amplifier in such systems.
A typical output stage of a class D amplifier is shown in FIG. 1 showing PMOS transistors 1 and 5 with input terminals A and C respectively and NMOS transistors 3 and 7 with input terminals B and D respectively. The pulse-width modulated signal is applied to each of the input terminals A, B, C and D. The junctions 9 and 11 of transistors are coupled together by a pair of filters, each filter containing an inductor 13, 15 and a capacitor 17, 19 with a load resistor 21 coupled between the filters. The output is taken from each of the junctions 9 and 11. When the input is positive, transistors 1 and 3 conduct longer and when the input is negative, transistors 5 and 7 conduct longer. When the differential is zero, both transistor pairs conduct with equal amount of time within a clock cycle. Therefore, the inductors 13 and 15 are required for filtering to avoid overdriving the load at high frequency.